Detector system having type of laser discrimination

ABSTRACT

Methods and apparatus for receiving a return laser pulse at a detector system having pixels in a pixel array and analyzing a response of the pixels in the pixel array including comparing the response to at least one threshold corresponding to decay of photonic energy of the laser pulse over distance and target reflectivity, wherein the at least one threshold comprises a first threshold corresponding to a low trigger for a pulse generated by a first type of laser and a second threshold corresponding to a high trigger for the pulse generated by the first type of laser. Embodiments can further include generating an alert signal based on the response of the pixels in the pixel array.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.17/197,328 filed Mar. 10, 2021 and entitled “DETECTOR SYSTEM COMPARINGPIXEL RESPONSE WITH PHOTONIC ENERGY DECAY,” the entire content of whichis incorporated herein by reference.

BACKGROUND

As is known in the art, some known ranging systems can include laserradar (ladar), light-detection and ranging (lidar), and rangefindingsystems, to measure the distance to objects in a scene. A laser rangingand imaging system emits a pulse toward a particular location andmeasures the return echoes to extract the range.

Conventional laser ranging systems generally work by emitting a laserpulse and recording the time it takes for the laser pulse to travel to atarget, reflect, and return to a photoreceiver. The laser ranginginstrument records the time of the outgoing pulse and records the timethat a laser pulse returns. The difference between these two times isthe time of flight to and from the target. Using the speed of light, theround-trip time of the pulses is used to calculate the distance to thetarget.

SUMMARY

Example embodiments of the disclosure provide methods and apparatus fora photonic integrated circuit, such as a readout integrated circuit(ROIC), having safety features, such as Automotive Safety IntegrityLevel (ASIL) related features. In embodiments, one or more features areembedded into a ROIC to enable ASIL compliance from a photoreceiver foruse in various applications, such as automotive applications.

A photonic system can include a detector pixel having thin conductivetraces to generate photonic energy at the sensitivity level of thephotonic system. In embodiments, a LED is located proximate the detectorpixel to generate a photon in response to an electrical signal. This LEDmay also be located within the package or even outside of thePhotoreceiver System that the Photonic ROIC is a part of. The ROIC wouldthen control the LED output directly. By generating photons directly,the entire signal path of the photoreceiver can be verified. In anotherembodiment, a voltage is applied to a trace to force a known voltage. Acurrent source can be used in place of the pixel.

In some embodiments, a ROIC generates an alert if a return pulse is notdetected within a certain time window versus the nominal response of notreturning data. In one embodiment, a ROIC applies a known signal to thepixel or a pixel array and waits for a response from the pixel andcircuit in the ROIC. In an embodiment, a ROIC can include pulsevalidation. For example, if the amplitude of a returned pulse versustime is not within an expected range, an alert can be generated. Inembodiments, a shape of a pulse can be evaluated. For example, the shapeof a pulse can be confirmed to match an expected shape to eliminatepotentially spurious pulses from the overall environment.

In embodiments, a ROIC can include direct electrical stimulation throughcurrent generation to validate functionality from transimpedanceamplifier (TIA) through back-end. Dynamic photodetector voltage biasmodulation over time can be evaluated by the ROIC on a low side tovalidate current response to be consistent with a properly functioningphotodetector. In some embodiments, a ROIC can include an ASIL signaloutput on a separate pin or pins of the package.

In one aspect, a method comprises: controlling a stimulus source todirect photons to a pixel in a pixel array contained in a detectorsystem; analyzing a response of the pixel in the pixel array; andgenerating an alert based on the response of the pixel in the pixelarray.

A method can further include one or more of the following features: thestimulus source comprises a metal object and controlling the stimulussource comprises heating the metal object, the stimulus source comprisesa current source, the stimulus source comprises a PN junction providinga light emitting diode (LED), analyzing the response of the pixel in thepixel array comprises determining that a response was not generated witha given period of time corresponding to a given distance, generating thealert based on the response of the pixel in the pixel array correspondsto the determining that the response was not generated with the givenperiod of time corresponding to the given distance, analyzing theresponse of the pixel in the pixel array comprises determining that theresponse was not generated with the given period of time correspondingto the given distance after the stimulus source to direct photons to thepixel in the pixel array was controlled to stimulate the pixel andgenerate the response, the detector system comprises a photodiodecoupled to an amplifier which provide an output to a comparator, thecomparator comprises a digital circuit, the detector system furthercomprises a first voltage threshold coupled to an input of thecomparator and an output of the comparator is used to analyze theresponse of the pixel in the pixel array, the detector array furthercomprises a multiplexer to multiplex an output of the pixel and a testsignal, wherein an output of the multiplexer is coupled to theamplifier, a readout integrated circuit performs the controlling of thestimulus source, the readout integrated circuit is external to thedetector array, and/or a readout integrated circuit controls thestimulus source to direct the photons to the pixels in the pixel arrayat a selected time.

In another aspect, a detector system comprises: a stimulus source todirect photons to pixels in a pixel array contained in the detectorsystem; a first module to analyze a response of the pixels in the pixelarray; and a second module to generate an alert based on the response ofthe pixels in the pixel array.

A system can further include one or more of following features: thestimulus source comprises a metal object that can be heated, thestimulus source comprises a current source, the stimulus sourcecomprises a PN junction providing a light emitting diode (LED), thefirst module is configured to analyze the response of the pixels in thepixel array by determining that a response was not generated with agiven period of time corresponding to a given distance, the alert isgenerated based on the response of the pixels in the pixel array bydetermining that the response was not generated with the given period oftime corresponding to the given distance, the first module is configuredto analyze the response of the pixels in the pixel array by determiningthat the response was not generated with the given period of timecorresponding to the given distance after the stimulus source to directphotons to the pixels in the pixel array was controlled to stimulate thepixel and generate the response, the detector system comprises aphotodiode coupled to an amplifier which provides an output to acomparator, the comparator comprises a digital circuit, the detectorsystem further comprises a first voltage threshold coupled to an inputof the comparator and an output of the comparator is used to analyze theresponse of the pixels in the pixel array, the detector array furthercomprises a multiplexer to multiplex an output of the pixels and a testsignal, wherein an output of the multiplexer is coupled to theamplifier, a readout integrated circuit is configured to control thestimulus source, the readout integrated circuit is external to thedetector array, and/or a readout integrated circuit controls thestimulus source to direct the photons to the pixels in the pixel arrayat a selected time.

In a further aspect, a method comprises: receiving a return laser pulseat a detector system having pixels in a pixel array; analyzing aresponse of the pixels in the pixel array including comparing theresponse to at least one threshold corresponding to decay of photonicenergy of the laser pulse over distance and target reflectivity; andgenerating an alert signal based on the response of the pixels in thepixel array.

A method can further include one or more of the following features: areadout integrated circuit controls a laser that generates the returnlaser pulse, the at least one threshold comprises a first thresholdcorresponding to a first reflectivity and a second thresholdcorresponding to a second reflectivity, analyzing the response comprisesdetermining that real return for the laser pulse corresponds to theresponse of the pixels in the pixel array being between the first andsecond thresholds, analyzing the response comprises determining thatnoise corresponds to the response of the pixels in the pixel array beingbelow the second threshold, analyzing the response comprises determiningthat noise corresponds to the response of the pixels in the pixel arraybeing above the first threshold, the at least one threshold comprises afirst threshold corresponding to a low trigger for a pulse generated bya first type of laser and a second threshold corresponding to a hightrigger for the pulse generated by the first type of laser, determininga first time from the returned laser pulse exceeding the first thresholdto the returned pulse exceeding the second threshold, determining basedon the first time that the returned laser pulse was generated by asecond type of laser that is different from the first type of laser,determining that the returned laser pulse was generated by a second typeof laser that is different from the first type of laser based upon apulse width of the returned laser pulse, the first time corresponds tothe laser type comprising a fiber laser, and/or the first timecorresponds to the laser type comprising a DPSS laser.

In a further aspect, a detector system comprises: a detector to receivea return laser pulse, wherein the detector comprises pixels in a pixelarray; a first module configured to analyze a response of the pixels inthe pixel array including comparing the response to at least onethreshold corresponding to decay of photonic energy of the laser pulseover distance and target reflectivity; and an alert signal configured togenerate an alert based on the response of the pixels in the pixelarray.

A system can further include one or more of the following features: areadout integrated circuit is configured to control a laser thatgenerates the return laser pulse, the at least one threshold comprises afirst threshold corresponding to a first reflectivity and a secondthreshold corresponding to a second reflectivity, analyzing the responsecomprises determining that real return for the laser pulse correspondsto the response of the pixels in the pixel array being between the firstand second thresholds, analyzing the response comprises determining thatnoise corresponds to the response of the pixels in the pixel array beingbelow the second threshold, analyzing the response comprises determiningthat noise corresponds to the response of the pixels in the pixel arraybeing above the first threshold, the at least one threshold comprises afirst threshold corresponding to a low trigger for a pulse generated bya first type of laser and a second threshold corresponding to a hightrigger for the pulse generated by the first type of laser, the systemis further configured to determine a first time from the returned laserpulse exceeding the first threshold to the returned pulse exceeding thesecond threshold, the system is further configured to determine based onthe first time that the returned laser pulse was generated by a secondtype of laser that is different from the first type of laser, the systemis further configured to determine that the returned laser pulse wasgenerated by a second type of laser that is different from the firsttype of laser based upon a pulse width of the returned laser pulse, thefirst time corresponds to the laser type comprising a fiber laser,and/or the first time corresponds to the laser type comprising a DPSSlaser.

In another aspect, a method comprises: employing an amplifier to amplifysignals from a photodetector forming part of a detector system having apixel array; applying an AC modulation signal to the amplifier, whereinan output signal of the amplifier includes a pulse signal generated bypixels in a pixel array contained in a detector system and the ACmodulation signal; analyzing an output signal from the amplifier todetect a fault in operation of the photodetector; and filtering the ACmodulation signal from the output signal of the amplifier. A method canfurther include one or more of the following features: evaluatingoperation of the photodetector, and/or generating an alert based on theevaluating of the photodetector.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of this disclosure, as well as the disclosureitself, may be more fully understood from the following description ofthe drawings in which:

FIG. 1 is a high level block diagram of an example detection systemhaving safety features;

FIG. 2 is a schematic representation of a portion of pixels in a pixelarray with pixel stimulation;

FIG. 3 is a representation of example circuit and operation of pixelresponse and signal timeout;

FIG. 3A is an example circuit diagram for pixel response and signaltimeout with current source stimulation;

FIG. 3B is an example block diagram for the circuit of FIG. 3A;

FIG. 4 is a waveform diagram of photonic energy return and voltagethresholds for reflectivity values;

FIG. 4A is an example circuit implementation for discriminating realsignal return using the reflectivity values of FIG. 4 ;

FIG. 4B is an example circuit implementation for generating the voltagethresholds of FIG. 4 ;

FIG. 5 is a waveform diagram showing laser pulse characteristics fordifferent laser types that can be used to validate signals;

FIG. 6 is an example circuit implementation of a dynamic photodetectorbias modulation;

FIG. 6A is an example of a modulated signal;

FIG. 6B is an example of a Photonic signal; and

FIG. 7 is a schematic representation of an example computer that canperform at least a portion of the processing described herein.

DETAILED DESCRIPTION

Prior to describing example embodiments of the disclosure someinformation is provided. Laser ranging systems can include laser radar(ladar), light-detection and ranging (lidar), and rangefinding systems,which are generic terms for the same class of instrument that uses lightto measure the distance to objects in a scene. This concept is similarto radar, except optical signals are used instead of radio waves.Similar to radar, a laser ranging and imaging system emits an opticalsignal, e.g. a pulse or continuous optical signal, toward a particularlocation and measures the return echoes to extract the range.

Laser ranging systems generally work by emitting a laser pulse andrecording the time it takes for the laser pulse to travel to a target,reflect, and return to a photoreceiver. The laser ranging instrumentrecords the time of the outgoing pulse—either from a trigger or fromcalculations that use measurements of the scatter from the outgoinglaser light—and then records the time that a laser pulse returns. Thedifference between these two times is the time of flight to and from thetarget. Using the speed of light, the round-trip time of the pulses isused to calculate the distance to the target.

Lidar systems may scan the beam across a target area to measure thedistance to multiple points across the field of view, producing a fullthree-dimensional range profile of the surroundings. More advanced flashlidar cameras, for example, contain an array of detector elements, eachable to record the time of flight to objects in their field of view.

When using light pulses to create images, the emitted pulse mayintercept multiple objects, at different orientations, as the pulsetraverses a 3D volume of space. The echoed laser-pulse waveform containsa temporal and amplitude imprint of the scene. By sampling the lightechoes, a record of the interactions of the emitted pulse is extractedwith the intercepted objects of the scene, allowing an accuratemulti-dimensional image to be created. To simplify signal processing andreduce data storage, laser ranging and imaging can be dedicated todiscrete-return systems, which record only the time of flight (TOF) ofthe first, or a few, individual target returns to obtainangle-angle-range images. In a discrete-return system, each recordedreturn corresponds, in principle, to an individual laser reflection(i.e., an echo from one particular reflecting surface, for example, avehicle, a person, a tree, pole or building). By recording just a fewindividual ranges, discrete-return systems simplify signal processingand reduce data storage, but they do so at the expense of lost targetand scene reflectivity data. Because laser-pulse energy has significantassociated costs and drives system size and weight, recording the TOFand pulse amplitude of more than one laser pulse return per transmittedpulse, to obtain angle-angle-range-intensity images, increases theamount of captured information per unit of pulse energy. All otherthings equal, capturing the full pulse return waveform offerssignificant advantages, such that the maximum data is extracted from theinvestment in average laser power. In full-waveform systems, eachbackscattered laser pulse received by the system is digitized at a highsampling rate (e.g., 500 MHz to 1.5 GHz). This process generatesdigitized waveforms (amplitude versus time) that may be processed toachieve higher-fidelity 3D images.

Of the various laser ranging instruments available, those withsingle-element photoreceivers generally obtain range data along a singlerange vector, at a fixed pointing angle. This type of instrument—whichis, for example, commonly used by golfers and hunters—either obtains therange (R) to one or more targets along a single pointing angle orobtains the range and reflected pulse intensity (I) of one or moreobjects along a single pointing angle, resulting in the collection ofpulse range-intensity data, (R,I)_(i), where i indicates the number ofpulse returns captured for each outgoing laser pulse.

More generally, laser ranging instruments can collect ranging data overa portion of the solid angles of a sphere, defined by two angularcoordinates (e.g., azimuth and elevation), which can be calibrated tothree-dimensional (3D) rectilinear cartesian coordinate grids; thesesystems are generally referred to as 3D lidar and ladar instruments. Theterms “lidar” and “ladar” are often used synonymously and, for thepurposes of this discussion, the terms “3D lidar,” “scanned lidar,” or“lidar” are used to refer to these systems without loss of generality.3D lidar instruments obtain three-dimensional (e.g., angle, angle,range) data sets. Conceptually, this would be equivalent to using arangefinder and scanning it across a scene, capturing the range ofobjects in the scene to create a multi-dimensional image. When only therange is captured from the return laser pulses, these instruments obtaina 3D data set (e.g., angle, angle, range)_(n), where the index n is usedto reflect that a series of range-resolved laser pulse returns can becollected, not just the first reflection.

Some 3D lidar instruments are also capable of collecting the intensityof the reflected pulse returns generated by the objects located at theresolved (angle, angle, range) objects in the scene. When both the rangeand intensity are recorded, a multi-dimensional data set [e.g., angle,angle, (range-intensity)_(n)] is obtained. This is analogous to a videocamera in which, for each instantaneous field of view (FOV), eacheffective camera pixel captures both the color and intensity of thescene observed through the lens. However, 3D lidar systems, insteadcapture the range to the object and the reflected pulse intensity.

Lidar systems can include different types of lasers, including thoseoperating at different wavelengths, including those that are not visible(e.g., those operating at a wavelength of 840 nm or 905 nm), and in thenear-infrared (e.g., those operating at a wavelength of 1064 nm or 1550nm), and the thermal infrared including those operating at wavelengthsknown as the “eyesafe” spectral region (i.e., generally those operatingat a wavelength beyond about 1400-nm), where ocular damage is lesslikely to occur. Lidar transmitters are generally invisible to the humaneye. However, when the wavelength of the laser is close to the range ofsensitivity of the human eye—roughly 350 nm to 730 nm—the energy of thelaser pulse and/or the average power of the laser must be lowered suchthat the laser operates at a wavelength to which the human eye is notsensitive. Thus, a laser operating at, for example, 1550 nm, can—withoutcausing ocular damage—generally have 200 times to 1 million times morelaser pulse energy than a laser operating at 840 nm or 905 nm.

One challenge for a lidar system is detecting poorly reflective objectsat long distance, which requires transmitting a laser pulse with enoughenergy that the return signal reflected from the distant target—is ofsufficient magnitude to be detected. To determine the minimum requiredlaser transmission power, several factors must be considered. Forinstance, the magnitude of the pulse returns scattering from the diffuseobjects in a scene is proportional to their range and the intensity ofthe return pulses generally scales with distance according to1/R{circumflex over ( )}4 for small objects and 1/R{circumflex over( )}2 for larger objects; yet, for highly-specularly reflecting objects(i.e., those objects that are not diffusively-scattering objects), thecollimated laser beams can be directly reflected back, largelyunattenuated. This means that—if the laser pulse is transmitted, thenreflected from a target 1 meter away—it is possible that the full energy(J) from the laser pulse will be reflected into the photoreceiver;but—if the laser pulse is transmitted, then reflected from a target 333meters away—it is possible that the return will have a pulse with energyapproximately 10{circumflex over ( )}12 weaker than the transmittedenergy.

In many cases of lidar systems highly-sensitive photoreceivers are usedto increase the system sensitivity to reduce the amount of laser pulseenergy that is needed to reach poorly reflective targets at the longestdistances required, and to maintain eyesafe operation. Some variants ofthese detectors include those that incorporate photodiodes, and/or offergain, such as avalanche photodiodes (APDs) or single-photon avalanchedetectors (SPADs). These variants can be configured as single-elementdetectors,-segmented-detectors, linear detector arrays, or area detectorarrays. Using highly sensitive detectors such as APDs or SPADs reducesthe amount of laser pulse energy required for long-distance ranging topoorly reflective targets. The technological challenge of thesephotodetectors is that they must also be able to accommodate theincredibly large dynamic range of signal amplitudes.

As dictated by the properties of the optics, the focus of a laser returnchanges as a function of range; as a result, near objects are often outof focus. Furthermore, also as dictated by the properties of the optics,the location and size of the “blur”—i.e., the spatial extent of theoptical signal—changes as a function of range, much like in a standardcamera. These challenges are commonly addressed by using largedetectors, segmented detectors, or multi-element detectors to captureall of the light or just a portion of the light over the full-distancerange of objects. It is generally advisable to design the optics suchthat reflections from close objects are blurred, so that a portion ofthe optical energy does not reach the detector or is spread betweenmultiple detectors. This design strategy reduces the dynamic rangerequirements of the detector and prevents the detector from damage.

Acquisition of the lidar imagery can include, for example, a 3D lidarsystem embedded in the front of car, where the 3D lidar system, includesa laser transmitter with any necessary optics, a single-elementphotoreceiver with any necessary dedicated or shared optics, and anoptical scanner used to scan (“paint”) the laser over the scene.Generating a full-frame 3D lidar range image—where the field of view is20 degrees by 60 degrees and the angular resolution is 0.1 degrees (10samples per degree)—requires emitting 120,000 pulses[(20*10*60*10)=120,000)]. When update rates of 30 frames per second arerequired, such as is required for automotive lidar, roughly 3.6 millionpulses per second must be generated and their returns captured.

There are many ways to combine and configure the elements of the lidarsystem— including considerations for the laser pulse energy, beamdivergence, detector array size and array format (single element,linear, 2D array), and scanner to obtain a 3D image. If higher powerlasers are deployed, pixelated detector arrays can be used, in whichcase the divergence of the laser would be mapped to a wider field ofview relative to that of the detector array, and the laser pulse energywould need to be increased to match the proportionally larger field ofview. For example— compared to the 3D lidar above—to obtainsame-resolution 3D lidar images 30 times per second, a 120,000-elementdetector array (e.g., 200×600 elements) could be used with a laser thathas pulse energy that is 120,000 times greater. The advantage of this“flash lidar” system is that it does not require an optical scanner; thedisadvantages are that the larger laser results in a larger, heaviersystem that consumes more power, and that it is possible that therequired higher pulse energy of the laser will be capable of causingocular damage. The maximum average laser power and maximum pulse energyare limited by the requirement for the system to be eyesafe.

As noted above, while many lidar system operate by recording only thelaser time of flight and using that data to obtain the distance to thefirst target return (closest) target, some lidar systems are capable ofcapturing both the range and intensity of one or multiple target returnscreated from each laser pulse. For example, for a lidar system that iscapable of recording multiple laser pulse returns, the system can detectand record the range and intensity of multiple returns from a singletransmitted pulse. In such a multi-pulse lidar system, the range andintensity of a return pulse from a from a closer-by object can berecorded, as well as the range and intensity of later reflection(s) ofthat pulse—one(s) that moved past the closer-by object and laterreflected off of more-distant object(s). Similarly, if glint from thesun reflecting from dust in the air or another laser pulse is detectedand mistakenly recorded, a multi-pulse lidar system allows for thereturn from the actual targets in the field of view to still beobtained.

The amplitude of the pulse return is primarily dependent on the specularand diffuse reflectivity of the target, the size of the target, and theorientation of the target. Laser returns from close, highly-reflectiveobjects, are many orders of magnitude greater in intensity than theintensity of returns from distant targets. Many lidar systems requirehighly sensitive photodetectors, for example avalanche photodiodes(APDs), which along with their CMOS amplification circuits allow lowreflectivity targets to be detected, provided the photoreceivercomponents are optimized for high conversion gain. Largely because oftheir high sensitivity, these detectors may be damaged by very intenselaser pulse returns.

However, capturing the intensity of pulses over a larger dynamic rangeassociated with laser ranging may be challenging because the signals aretoo large to capture directly. One can infer the intensity by using arecording of a bit-modulated output obtained using serial-bit encodingobtained from one or more voltage threshold levels. This technique isoften referred to as time-over-threshold (TOT) recording or, whenmultiple-thresholds are used, multiple time-over-threshold (MTOT)recording.

FIG. 1 shows an example detector system 100 having safety functionality.A detector array 102, which can comprise a focal plane array (FPA) 105having an array of pixels, is coupled to a readout module 104, such as areadout integrated circuit (ROIC). Although the FPA 105 is shown as aROIC and detector array in another embodiment they may comprise onepiece of material, for example a silicon FPA. In addition, the READOUTmodule 106 may comprise a silicon circuit and the detector module 102may comprises a different material, such as, but not limited to GaAs,InGaAs, InGaAsP, and/or other detector materials.

In embodiments, the detector array 102 can comprise a single pixel, orpixels in one dimension (1D, two dimensions (2D), and/or threedimensions (3D) arrays). An interface module 106 can output theinformation from the readout module 104. A safety module 108 can analyzeoperation of the detector system 100 and generate alerts upon detectingone or more faults. In embodiments, the safety module 108 can provideAutomotive Safety Integrity Level (ASIL) related functionality, asdescribed more fully below. The detector system 100 can include aregulator 110 to provide one or more regulated voltages for the system.

FIG. 2 shows a portion of a detector array 200 having a series of pixels202 proximate to a direct photon injection mechanism. Direct photoninjection refers to creating photonic energy in proximity to one or moreof the pixels 202 in order to stimulate a photonic response from thedetector array 200. By stimulating a known response, the full signalpath of a detector system can be validated.

In one embodiment, a conductive trace 204 can be heated to a temperaturethat causes photons to be emitted. Conductive trace 204 may be a traceon the detector array or a wire positioned in proximity to the detectorarray in a package. The conductive trace may be a conductor, such as ametal, including but not limited to aluminum, copper, tungsten, or amaterial such as indium tin oxide. The conductive trace 204 can beheated in a controlled manner, for example applying a known current to aconductive trace 204 or wire and time to emit photons in a particularway that results in an expected response from the detector array. If theexpected response is not detected, a fault can be detected.

While shown in the illustrated embodiment as aligned with centers of thepixels 202, a conductive trace 204 can be positioned in any practicallocation in relation to at least one pixel to meet the needs of aparticular application. In addition, the geometry of the conductivetrace 204 in thickness, length, height, shape etc., can vary. In someembodiments, the conductive trace 204 cross section can be cylindricallike a wire, or rectangular or trapezoidal as in a semiconductormetallization process.

In another embodiment, a PN junction 206 can be formed proximate thepixel(s) 202 to provide a light emitting diode (LED) 208) that emitsphotons. The PN junction 206 can be stimulated to emit photons in aparticular way that results in an expected response from the detectorarray. If the expected response is not detected, a fault can begenerated.

The PN junction 206 can be formed from any suitable material(s), such assilicon and non-silicon materials. In one embodiment, the PN junctioncomprises GaAs, InGaAs, or InGaAsP.

The source of stimulation may also be external to the detector array andROIC (the focal plane array) with the ROIC providing the control tosynchronize the timing and also controlling the amount of photonicstimulus. This stimulus may be included inside the package that the ROICand APD, an example of a focal plane array (FPA), reside within. In anembodiment the stimulus can comprise an external LED diode of any sizeor shape as long as the placement and the additional design of thepackage results in the desired level of photonic energy to stimulate thepixels 202. In an embodiment the ROIC may provide a signal to a lidarsystem laser in a lidar system to pulse or apply a known signal at agiven time.

This stimulation may interfere with the desired optical signal, as suchthis stimulus may be timed, but is not limited to these techniques, inone of the following ways: 1) Triggered upon an external stimulusdenoting a desired self check to be run. 2) Upon start-up of the deviceto ensure each time it powers up that it will function properly, or 3)after it has received an actual photonic return, some time after, orafter a timeout event, to enable testing after each pulse (a continuoustest mode)

In other embodiments, a current source 210 can replace, or bemultiplexed 212, with pixel response to simulate a response. In thisway, a signal path can be stimulated and validated. In some embodiments,a light emitting pixel 202 may be substituted for a current source, byreplacing a live pixel or multiplexing between the cathode or anode ofthe pixel. In another embodiment the light emitting pixel may be on thereadout circuit and direct light when stimulated toward the detectorarray in a focal plane array. In some cases, this may require thedetector array to be thin. Another light emitting die may be positionedseparate from the detector array or the readout circuit die. This threedie solution is not shown.

FIG. 3 shows a portion of a detector system 300, such as the system ofFIG. 1 , that can form part of, or be coupled to, a ROIC. It isunderstood that a detector system 300 can comprise a single pixel. Aphotodiode 302 is coupled to bias voltage 304, e.g., 60V. In theillustrated embodiment, a transimpedance amplifier (TIA) 306 is coupledto the cathode of the photodiode 302 and generates an output 308 that isprovide to an input of a comparator 310. A threshold voltage Vthprovides a second input to the comparator 310, which generates athreshold output signal 312. It is understood that a TIA refers to acurrent to voltage converter that can amplify the current from thephotodiode 302. An alert module 314 can generate alerts to indicate oneor more fault conditions.

In embodiments, a detector system can provide an alert at a specifiedmaximum range to indicate that a response was not received within acertain distance. In embodiments, normal operation would not generatealerts as pulses are received and detected. As such this is an “active”indication of non-response situations. This can be used where there areknown obstacles, either simulated through a fiber delay loop or actualand non-response within this interval indicates a problem with thesystem.

In the illustrated embodiment, the output of the amplifier 308 generatesa pulse 320 generated by current from the by the photodiode 304 inresponse to photon detection. The pulse 320 has an amplitude that isabove the voltage threshold Vth at the input of the comparator 310.Within some maximum time window 322, which corresponds to a maximumdistance, the output 312 of the comparator should change state. If thecomparator output 312 does not transition, an alert can be generated bythe alert module 314. A trigger can correspond to no response within aset time 324 or window.

To validate detection operation, a pixel can be manipulated, or photonicstimulus generated by a heat element, LED, current source, etc., todetermine whether the pixel circuit does, or does not, generate an alertwhen a fault occurs. In the illustrated embodiment, a test pulse 326 canbe generated to test operation of the circuit and alert generation. Forexample, if there is an expectation a ‘hit’ should occur before acertain distance a time out signal, e.g., no transition of thecomparator output 312, shows a malfunction. In embodiments, anypractical time out can be set to meet the requirements of a particularapplication.

FIG. 3A shows an example circuit implementation similar to the circuit300 of FIG. 3 with the addition of a multiplexer 340 having a firstinput from the photodiode 302 and a second input from a current source342. The selected input of the multiplexer 340 can be provided to theamplifier 306. A stimulus can be generated by the current source 342 toevaluate circuit operation and alert generation, as described above.

FIG. 3B shows an example block diagram having a photodetection circuit350 receiving an input from a stimulus module 352, which can include acurrent source, and generating an output to a threshold detector module354. For example, the stimulus module 352 can generate the test pulse326 in FIG. 3 . The threshold detector module 354 can validate operationof the circuit in response to selected stimuli. For example, asdescribed above, the threshold detector module 354 can evaluate signalsagainst one or more thresholds. A ROIC can include an external outputsignal 356 that can provide an alert. In embodiments, the output signal356 can include one or more ASIL signals that can be connected to aremote system, such as an engine control unit (ECU), an obstacledetection controller, vehicle control unit, vehicle control system, orvehicle computer.

FIG. 4 shows a detection system that includes functionality to reducefalse detections. A first curve 400 shows amplitude over time for 90%reflectivity for a given target and a second curve 402 shows 10%reflectivity. The first curve 400 corresponds to a first voltagethreshold Vth1 and the second curve 402 corresponds to a second voltagethreshold Vth2.

In the illustrative embodiment, voltage pulses 410, 412 between thefirst and second voltage thresholds Vth1, Vth2, are generated by alikely real return. A voltage pulse 414 below the second voltagethreshold Vth2 is likely noise. A voltage pulse 416 above the firstvoltage threshold Vth1 is likely noise.

As can be seen, decay of the returned photonic energy vs. distance ismodulated by reflectivity. A range of reflectivities can be selectedbased on the characteristics of the transmitted pulses, expected targetcharacteristics, expected distances, and the like. The detector can becalibrated with an actual source and the response energy can be modeledfor a reasonable range of response over time. This increases safety byimproving false pulse rejection. In addition, real pulses can be betterdiscerned.

FIG. 4A shows an example circuit implementation 450 including aphotodiode 452 providing an input to an amplifier 454 generating anoutput that is coupled to inputs of first and second comparators 456,458. In the illustrated embodiment, a 60V bias voltage 459 is applied tothe photodiode 452. It is understood that any practical bias voltagelevel can be used. A first voltage threshold Vth1 is coupled to a secondinput of the first comparator 456 and a second voltage threshold Vth2 iscoupled to a second input of the second comparator 458. The outputs ofthe first and second comparators 456, 458 are provided as inputs to anAND gate 460, which changes state when the output of the amplifier 454is between the first and second voltage thresholds Vth1, Vth2 inaccordance with the first and second curves 400, 402 of FIG. 4 , forexample.

FIG. 4B shows an example implementation in which the first voltagethreshold Vth1 is generated by a high speed digital-to-analog converter(DAC) or a DAC setting a decaying RC circuit.

FIG. 5 shows an example plot of a first laser pulse 500 generated by afirst type of laser, such as a fiber laser, and a second laser pulse 550generated by a second type of laser, such as a diode pumped solid state(DPSS) laser. Each of the laser pulses 500, 550 have different patternsby which the energy is emitted. The first pulse 500 is a shorter andsharper pulse of a set time and the second pulse 550 is a longer/widerpulse with a shallower rise and steeper fall. The characteristics of thetransmitted laser pulses 500, 550 can be used to enhance detection oflower energy pulses, and can also reduce erroneous detection of pulsesthat do not conform to the pulse characteristics.

The first laser pulse 500 can be compared to a low trigger threshold 502and a high trigger threshold 504 to time the duration of the pulse,e.g., the time to cross the thresholds 502, 504 going up (rise) to thetime to cross going down (fall). Pulses that do not conform (withinmargins for distance and pulse reflectivity) and/or meet certain ratiocharacteristics between durations can be rejected. Relatively lowerenergy pulses can be detected. In embodiments, thresholds similar to thethresholds Vth1, Vth2 of FIG. 4 can be used for the High Trigger and LowTrigger illustrated in FIG. 5 and similar circuitry as that shown inFIG. 4A can be used to process received pulses.

As can be seen, the DPSS laser pulse 550 has a leaky period before thelaser fires that can also be timed against the durations for the highand low trigger and compared to one another.

For example, if a detector expects to receive pulses of the first type500 pulses of the second type 502 can be discriminated, e.g., rejectedas noise. In embodiments, a detector can reject pulses that are not ofthe expected type. For example, in automotive applications there may bea number of devices transmitting pulse of various types. Bydiscriminating pulses from other types of lasers by pulse shape, falsedetections can be reduced.

In embodiments, pulse characteristics can be evaluated, for example, bydesign, where through manufacturing properties are understood, orcharacterized per unit using an offline characterization, or by using afiber delay loop or target at a known distance with known reflectivity.

FIG. 6 shows an example circuit implementation of a part of aphotodetector having bias modulation to detect circuit malfunction. Aphotodetector can be biased on two terminals, such as a common cathode(coupled to other cathodes) at a higher external voltage and an internalpoint at a lower voltage that is below a current measuring circuit,e.g., a TIA. The lower bias point is modulated to generate a knownsignal at a known frequency. Certain characteristics of the response arebased on the bias, such as “dark-current”, but also gain and othercharacteristics. By coupling to and measuring the signal at the knowninjection frequency, one can monitor the health and function of thephotodiode by comparing this response at the known frequency to theexpected response. Deviation from this response can be used to trigger asafety condition where the photodetector is considered to not beoperating properly.

In an example embodiment, a photodiode 600 has a cathode coupled to abias voltage source 602 and an anode coupled to the input of anamplifier 604, such as a TIA. An AC modulator 606 is coupled to theamplifier 604 so that the output of the amplifier is modulated by thesignal from the modulator. A high pass filter 614 can filter out themodulation signal.

As can be seen in FIG. 6A, the modulated signal from the output of theamplifier 604 includes a pulse 610 from the photodiode 600 and amodulation signal amplitude 612 matching an expected output generated bythe AC modulation signal from the modulator 606. The modulation signalin the amplifier 604 output is indicative of proper operation of thecircuit. As shown in FIG. 6B, a photonic output can be generated afterthe high pass filter (set above the modulation frequency) 614 filtersthe amplifier output to remove the modulation signal. With thisarrangement, photodiode 600 function can be checked. A signalcomparison/evaluation module 613 can compare expected signals to actualsignals to detect faults and/or generate alerts.

In other embodiments, a signal from the bias voltage source 602 can bemodulated through the photodiode 600. By detecting the modulated signalpulses/amplitude at the output of the amplifier 604, operation of thephotodiode 600 can be checked.

FIG. 7 shows an exemplary computer 700 or controller that can perform atleast part of the processing described herein. For example, the computer700 can perform processing to implement a mask controller, such as theselect module 214 of FIG. 2 , for example, as well as the steps in FIG.5 . The computer 700 includes a processor 702, a volatile memory 704, anon-volatile memory 706 (e.g., hard disk, or other memory such as FLASH,EEPROM, or RAM), an output device 707 and a voice control unit, and/or agraphical user interface (GUI) 708 (e.g., a mouse, a keyboard, adisplay, for example). The non-volatile memory 706 stores computerinstructions 712, an operating system 716 and data 718. In one example,the computer instructions 712 are executed by the processor 702 out ofvolatile memory 704. In one embodiment, an article 720 comprisesnon-transitory computer-readable instructions.

Processing may be implemented in hardware, software, or a combination ofthe two. Processing may be implemented in computer programs executed onprogrammable computers/machines that each includes a processor, astorage medium or other article of manufacture that is readable by theprocessor (including volatile and non-volatile memory and/or storageelements), at least one input device, and one or more output devices.Program code may be applied to data entered using an input device toperform processing and to generate output information.

The system can perform processing, at least in part, via a computerprogram product, (e.g., in a machine-readable storage device), forexecution by, or to control the operation of, data processing apparatus(e.g., a programmable processor, a computer, or multiple computers).Each such program may be implemented in a high-level procedural orobject-oriented programming language to communicate with a computersystem. However, the programs may be implemented in assembly or machinelanguage. The language may be a compiled or an interpreted language andit may be deployed in any form, including as a stand-alone program or asa module, component, subroutine, or other unit suitable for use in acomputing environment. A computer program may be deployed to be executedon one computer or on multiple computers at one site or distributedacross multiple sites and interconnected by a communication network. Acomputer program may be stored on a storage medium or device (e.g.,RAM/ROM, including FLASH memory, or EEPROM, CD-ROM, hard disk, ormagnetic diskette) that is readable by a general or special purposeprogrammable computer for configuring and operating the computer whenthe storage medium or device is read by the computer.

Processing may also be implemented as a machine-readable storage medium,configured with a computer program, where upon execution, instructionsin the computer program cause the computer to operate.

Processing may be performed by one or more programmable processorsexecuting one or more computer programs to perform the functions of thesystem. All or part of the system may be implemented as, special purposelogic circuitry (e.g., an FPGA (field programmable gate array), ageneral purpose graphical processing units (GPGPU), and/or an ASIC(application-specific integrated circuit)).

Having described exemplary embodiments of the disclosure, it will nowbecome apparent to one of ordinary skill in the art that otherembodiments incorporating their concepts may also be used. Theembodiments contained herein should not be limited to disclosedembodiments but rather should be limited only by the spirit and scope ofthe appended claims. All publications and references cited herein areexpressly incorporated herein by reference in their entirety.

Elements of different embodiments described herein may be combined toform other embodiments not specifically set forth above. Variouselements, which are described in the context of a single embodiment, mayalso be provided separately or in any suitable subcombination. Otherembodiments not specifically described herein are also within the scopeof the following claims.

What is claimed is:
 1. A method, comprising: receiving a return laserpulse at a detector system having pixels in a pixel array; analyzing aresponse of the pixels in the pixel array including comparing theresponse to at least one threshold corresponding to decay of photonicenergy of the laser pulse over distance and target reflectivity, whereinthe at least one threshold comprises a first threshold corresponding toa low trigger for a pulse generated by a first type of laser and asecond threshold corresponding to a high trigger for the pulse generatedby the first type of laser; and generating an alert signal based on theresponse of the pixels in the pixel array.
 2. The method according toclaim 1, wherein a readout integrated circuit controls a laser thatgenerates the return laser pulse.
 3. The method according to claim 1,wherein the at least one threshold comprises a third thresholdcorresponding to a first reflectivity and a fourth thresholdcorresponding to a second reflectivity.
 4. The method according to claim3, wherein analyzing the response comprises determining that real returnfor the laser pulse corresponds to the response of the pixels in thepixel array being between the third and fourth thresholds.
 5. The methodaccording to claim 4, wherein analyzing the response comprisesdetermining that noise corresponds to the response of the pixels in thepixel array being below the fourth threshold.
 6. The method according toclaim 5, wherein analyzing the response comprises determining that noisecorresponds to the response of the pixels in the pixel array being abovethe third threshold.
 7. The method according to claim 1, furtherincluding determining a first time from the returned laser pulseexceeding the first threshold to the returned pulse exceeding the secondthreshold.
 8. The method according to claim 7, further includingdetermining based on the first time that the returned laser pulse wasgenerated by a second type of laser that is different from the firsttype of laser.
 9. The method according to claim 8, further includingdetermining that the returned laser pulse was generated by a second typeof laser that is different from the first type of laser based upon apulse width of the returned laser pulse.
 10. The method according toclaim 7, wherein the first time corresponds to the laser type comprisinga fiber laser.
 11. The method according to claim 7, wherein the firsttime corresponds to the laser type comprising a DPSS laser.
 12. Adetector system, comprising a detector to receive a return laser pulse,wherein the detector comprises pixels in a pixel array; a first moduleconfigured to analyze a response of the pixels in the pixel arrayincluding comparing the response to at least one threshold correspondingto decay of photonic energy of the laser pulse over distance and targetreflectivity, wherein the at least one threshold comprises a firstthreshold corresponding to a low trigger for a pulse generated by afirst type of laser and a second threshold corresponding to a hightrigger for the pulse generated by the first type of laser; and an alertsignal configured to generate an alert based on the response of thepixels in the pixel array.
 13. The system according to claim 12, whereina readout integrated circuit is configured to control a laser thatgenerates the return laser pulse.
 14. The system according to claim 12,wherein the at least one threshold comprises a third thresholdcorresponding to a first reflectivity and a fourth thresholdcorresponding to a second reflectivity.
 15. The system according toclaim 14, wherein analyzing the response comprises determining that realreturn for the laser pulse corresponds to the response of the pixels inthe pixel array being between the third and fourth thresholds.
 16. Thesystem according to claim 15, wherein analyzing the response comprisesdetermining that noise corresponds to the response of the pixels in thepixel array being below the fourth threshold.
 17. The system accordingto claim 16, wherein analyzing the response comprises determining thatnoise corresponds to the response of the pixels in the pixel array beingabove the third threshold.
 18. The system according to claim 12, whereinthe system is further configured to determine a first time from thereturned laser pulse exceeding the first threshold to the returned pulseexceeding the second threshold.
 19. The system according to claim 18,wherein the system is further configured to determine based on the firsttime that the returned laser pulse was generated by a second type oflaser that is different from the first type of laser.
 20. The systemaccording to claim 19, wherein the system is further configured todetermine that the returned laser pulse was generated by a second typeof laser that is different from the first type of laser based upon apulse width of the returned laser pulse.
 21. The system according toclaim 18, wherein the first time corresponds to the laser typecomprising a fiber laser.
 22. The system according to claim 18, whereinthe first time corresponds to the laser type comprising a DPSS laser.